More importantly the 261 rules were intended for just flights and what we are dealing with here is an amendment to a Package which is covered by different regulations, if the package is "rebuilt" before departure the 261 flight change rule may not be relevant. If the Tour Operator said in advance they were changing the flight content of a package it wouldn't be considered to be a 261 issue, but if it hadn't been changed and went wrong on the day then it would be covered.
As I said several posts ago, whether or not they can change a departure airport depends what is in the T&Cs. On their website under First Choice Sun Holidays, their T&Cs clearly state that a change of UK departure airport (other than swapping one London for another) would be considered as a Major Change. If they inform you less than 7 days before departure they will give you £40 per person in the party (have they told you that?) before you even get round to the what-happens-next stage. If you don't like the change you have the option of a completely different holiday of similar value or a full refund of all money paid. That's the contract terms
they set so there shouldn't be any argument about which law counts. It's possible that different brochures may have slighty different T&Cs but I suspect the Major Change section will be the same. You should have checked yours before even going to the shop because it would have saved a lot of hassle. It's standard travel trade practise to fob you off because most people don't read the T&Cs.
Going back to the 261 rules, UK courts are refusing to even accept some claims for listing as some sections are being refered back to Strasbourg for clarification, so for the moment it's best to have a Plan B. There is no such thing as EU law, just EU standards which each country is required to adopt into their own legislation
as a minimum. Which may explain why in another thread a member from Ireland got better treatment from Aer Lingus than various UK members with the same problem of snow disruption at the end of last year.